Electrophysiological and optical recording techniques are used primarily to elucidate the development, differentiation and cellular distribution of physiologically important properties expressed by vertebrate CNS neurons. Electrical studies involve direct, high-fidelity amplification of ion fluxes generated either in single cells or patches or in synaptically coupled pairs of cells maintained in monolayer culture. Optical recordings include indirect measurements of membrane potential or of intracellular ion concentration in small populations (50-100) of cultured cells. Principal findings this year include: 1) Nao+-dependent action potentials and underlying voltage-dependent Na+ and Cl- currents are expressed as early as embryonic (E) day 12 in telencephalic neurons, when most of the cells are still actively proliferating; 2) electrical and dye coupling among embryonic cells in intact telencephalon is greatest at E12 and then decreases during embryogenesis; 3) micromolar GABA activates Cl- conductance with heterogenous properties in neuroepithelial cells from the spinal cord at E13; 4) GABA-activated Cl- channels are not markedly sensitive to classical antagonists of GABA at Cl- channels; 5) embryonic chick telencephalic cells initially exhibit depolarizing GABA receptors that decrease free cytoplasmic Cl- (Clc-) and increase free cytoplasmic Ca2+ (Cac2+) but after 1 week in culture, receptor activation leads to an increase in Clc- and decrease in Cac2+; 6) E17 rat spinal cord cells initially express depolarizing GABA receptors that are more effective than receptors recorded in postnatal cells in terms of dose-response characteristics and desensitizing properties; 7) initially GABA is released in a continuous, tonic manner from embryonic neurons before it mediates transient signals; 8) dynamic interconversion between tonic and transient forms of release is correlated with mechanisms of intracellular Ca2+ homeostasis; 9) GABA included in the intracellular recording saline generates tonic activation of Cl- channels in non-neuronal cells transfected with GABA receptor mRNAs; 10) GABA activates longer~lasting Cl- channels in neurons dissociated from the embryonic relative to postnatal and adult thalamus; 11) shortening of GABA-activated Cl- channels parallels changes in the intracellular Cl~ concentration; 12) the Cl- channel-activating effects of GABA applied to embryonic thalamic neurons long outlast the application period and are sensitive to pressure-applied saline but not to GABA uptake blockers.